The present invention relates to switching aid circuits of static switches in electrical power converters such as, for example, choppers or undulators.
Such static switches are in general made of transistors, simple thyristors, or G.T.O. (Gate turn off) thyristors.
A switching aid circuit is often associated to these components to limit the overvoltages and the voltage rise speed of their terminals at the moment of opening. In general, it houses a parallel connected capacity on the static switches which is loaded at each static switch opening and is unloaded in other moments.
The simplest switching aid circuit is made of a dipole which is parallel connected on the static switch and which includes the said capacity series connected with a diode located in the same direction as the static switch and shunted by a resistor. The diode constitutes the capacitor loading circuit whereas the resistor and the static switch form the capacitor unloading circuit.
This configuration has a major disadvantage: the power stored in the capacitor at each static switch opening is entirely dissipated in the resistor by joule effect at the moment of static switch closing and this entails deterioration of the power conversion efficiency. If such losses can easily be dissipated in static converters having relatively low power, it is not the case when the static switches must cut currents of several hundred (even thousands) ampers under voltages which can exceed one thousand volts. In addition, the losses in such devices being proportional to the switching frequency, this configuration can only be used for relatively low frequencies.
A second disadvantage consists in the fact that the rise speed of the capacity unloading current in the static switch is very high (this rise speed being only limited by spurious inductances of the installation) and provokes important switching losses.
Another more intricate configuration of a switching aid circuit allowing reduction of losses by recovery of the switching aid capacitor load, and especially a circuit used in an undulator, is described in page 1589 of the Intenational Conference on Power Electronics Report, Tokyo, April 1983. This circuit includes, in series with the resistor of the switching aid capacitor unloading circuit, the primary winding of a transformer of which the secondary winding is connected to an energy recovery circuit on the power source by means of a rectifier assembly.
The presence of the unloading resistor which remains mandatory to limit the duration of the primary winding current magnetizing the transformer when the load of the switching aid capacitor have been cancelled and when the transformer primary winding is looped on itself via the capacitor loading diode, deteriorates the recovery efficiency of such device.